In adhesive dispensing systems, a dispensing module is generally fed liquid adhesive material from an adhesive melter or another similar supply device. These adhesive melters receive a controlled supply of solid adhesive material in the form of pellets from a supply hopper or some other storage unit. In this regard, transfer pumps for solid materials are used to control the supply of pellets of adhesive from the supply hopper to the adhesive melter. More particularly, transfer pumps remove pellets of adhesive from the supply hopper and force the pellets of adhesive with pressurized air through an outlet hose and to the adhesive melter, where the pellets of adhesive are melted to a liquid state for delivery to the dispensing module. The pellets of adhesive utilized in these adhesive dispensing systems have various shapes and sizes, but conventional transfer pumps are limited in what size and shape of pellets can be moved through the outlet hose to the adhesive melter.
In one example of a transfer pump known as a Venturi pump, the transfer pump expels one or more air jets through a passage to form a vacuum force that draws pellets of adhesive from the supply hopper and through the transfer pump. However, Venturi pumps are generally limited to creating a vacuum force of up to one atmosphere, thereby limiting the size of pellets that may be effectively drawn through the transfer pump. Additionally, the passage through which the air jets are expelled must be designed with a narrowed throat portion carefully tailored to maximize the vacuum force produced by the Venturi pump. The diameter of this narrowed throat portion may constrict or clog flow of pellets of adhesive through the transfer pump, thereby limiting the maximum size of pellets of adhesive that are moveable with the air jets.
In another example of a transfer pump known as a gravity eductor, the transfer pump expels an air jet to push pellets of adhesive from an inlet of the transfer pump and through the transfer pump. The pellets of adhesive are gravity fed into the inlet of the transfer pump by the supply hopper. Although the force exerted by the air jet in a gravity eductor can move a large number of larger pellets of adhesive, the resultant higher density of material within the transfer pump may clog the transfer pump, especially when the air jet is stopped and then restarted. As a result, gravity eductor transfer pumps cannot be stopped during operation unless the supply hopper feeding the gravity eductor is empty or the supply hopper includes additional valve structure for cutting off the gravity feed of pellets into the transfer pump. This additional valve structure is expensive and complicated, so most gravity eductors do not include the valve structure and are thus not stopped until the supply hopper is empty.
Consequently, it would be desirable to address these and other concerns associated with conventional transfer pumps.